The present invention relates to a counter or an odometer capable of preventing tampering or the rotation in the subtractive direction.
There have been devised and demonstrated various types of tamper preventive odometers. Of these the most common type is such that a drive shaft which rotates in proportion to that of wheels of a vehicle carries a worm gear which is pressed under the force of a spring against a gear made integral with a least significant digit (LSD) wheel of an odometer in such a way that the rotation of the drive shaft may be transmitted to the least significant digit wheel in one or additive direction, but when the drive shaft is rotated in the opposite or subtractive direction, the consequent rotation in the other or subtractive direction of the worm gear causes the decrease in strength of the spring, resulting in the disengagement of the worm gear with the gear of the least significant digit gear. As a result, the rotation in the other or subtractive direction of the odometer may be prevented.
However, tamper preventive odometers of the type described have to use a worm gear which is relatively long, resulting in an increase in size. In addition, the spring-actuated clutch for engaging and disengaging between the worm gear and the gear carried by the least significant digit wheel is complicated in construction.